1. Field of the Invention
The present invention relates to a device for controlling a behavior of a vehicle such as an automobile (a stability control), and more specifically, to such a device that stabilizes the running behavior of a vehicle by controlling a steering angle of wheels.
2. Description of Prior Art
There have been developed various control devices for stabilizing a running behavior of a vehicle in its yaw direction through generating a counter yaw moment of suppressing oversteering and understeering tendencies owing to an imbalance in braking or traction forces in the lateral direction of the vehicle, the saturation of a tire force, etc. Such a counter yaw moment, usually, is generated by controlling the tire force distribution in a vehicle. Further, in a vehicle equipped with a steering system enabling the steering of wheels independently of a driver's steering action, a counter yaw moment can be generated by an automatic steering of wheels. Examples of control devices for stabilizing a vehicle behavior through an automatic steering are disclosed in Japanese Patent Laid-Open Publication No. 5-105055 and Japanese Patent No. 2540742, in which a steering angle of wheels is controlled to generate a counter yaw moment against the yaw moment owing to a braking force imbalance between the left and right wheels on a vehicle, and thereby ensuring its straight line stability.
In control devices as described above, a degree of deterioration of a vehicle behavior, i.e. oversteering and understeering tendencies, is monitored by comparing an actual or measured value of a parameter indicating a turning state, such as a yaw rate, with the corresponding target value determined with operational parameters, such as a steering angle and a vehicle speed. Then, a counter yaw moment is generated to reduce the difference between the actual and target values of the turning state parameter by adjusting individual braking forces and/or the steering angle of wheels.
In the above-mentioned control strategies, the execution of the automatic steering for the stability control alters an actual steering angle from an angle determined with a driver's steering operation (the rotational angle of a steering wheel operated by a driver). Under an extreme condition where a large counter yaw moment is required for stabilizing a vehicle behavior, not only the magnitude but also the direction of the actual steering angle may be altered from those determined with the driver's steering operation. However, this fact is not taken into account in conventional control devices as described above. In these devices, an actual steering angle is always used for determining a target value for the turning state parameter, so that, if the actual steering angle is altered by the automatic steering control, the resultant behavior and/or tracking course of the vehicle would be deviated from those intended by the driver.
Accordingly, conventional vehicle stability control devices as described above may be improved to operate more appropriately, taking into account a steering angle variation under the automatic steering control.